1. Field of the Invention
The present invention relates to an imaging optical system for imaging equipment and imaging equipment each of which uses a compact solid-state imaging device such as a CCD image sensor and a CMOS image sensor having a size of ½ inches or less.
2. Description of the Related Art
In imaging equipment using the solid-state imaging device, the imaging optical system in the imaging equipment for action camera or car-mounted camera is required to have a wider field angle and improved thermal characteristics other than a small size and lightness in weight and a high resolution. Furthermore, the imaging optical system of the car-mounted camera is also required to have a capability of being produced at a low cost and in large quantities and to allow for solder reflow mounting onto the imaging equipment. As a method for the mass production, it is preferable to be capable of dealing with a wafer level lens manufacturing method.
As an imaging lens which realizes a small size and lightness in weight and enables a mass production as described above, there is an imaging lens having a doublet-type lens group configuration or a triplet-type lens group configuration, in which the solder reflow can be used and thermal characteristics are improved by forming a resin lens on a flat glass plate or a glass lens (hybrid type) (refer to Japanese Patent No. 3929479 and Japanese Patent No. 3946245).
In addition, as a similar imaging lens, there is an imaging lens which realizes a wider field angle by means of arranging a thick glass-made concave lens having a strong negative power on the front side of a hybrid lens (refer to Japanese Patent Laid Open No. 2012-113149).
Furthermore, as a similar imaging lens, there is an imaging lens which has a fast wide lens while suppressing cost for forming a resin lens configuration of 4 groups 4 lenses (refer to Japanese Patent Laid Open No. 1997-015492).
Moreover, as a similar imaging lens, there is an imaging lens which can deal with a wider field angle, a higher resolution, and a lower cost by forming the first lens with a spherical glass lens in a configuration of 4 groups 5 lenses and by bonding two lenses to each other among the other four aspherical resin lenses (refer to Japanese Patent Laid Open No. 2009-063877).
In addition, as a similar imaging lens, there is an imaging lens in which a low cost and compact size wide lens is realized by forming the first lens with a spherical glass lens in a configuration of 4 groups 4 lenses and by constituting the other three lenses with the use of three aspherical resin lenses or two aspherical resin lenses and a spherical glass lens (refer to Japanese Patent Laid Open No. 2010-107820).
Furthermore, as a similar lens, there is an imaging lens which realizes an optical system having a compact-size and a low-cost production capability by a configuration using a small thickness lens (refer to Japanese Patent Laid Open No. 2012-053384).
However, since the imaging lens as disclosed in Japanese Patent No. 3929479 and Japanese Patent No. 3946245 is a hybrid type lens in which a plane parallel glass plate or a glass lens close to the plane parallel plate is used as a base in a lens system, the shape of the whole lens system is restricted by a base glass shape. Accordingly, for example, it is difficult to form the lens system as a wide lens having a short focal length. In the examples shown in Japanese Patent No. 3929479 and Japanese Patent No. 3946245, the field angle (semi-field angle) w is approximately 32 degrees and the field angle is not sufficient to be applied to a wide lens of a car-mounted camera or the like.
While a wider field angle is realized by arranging a glass lens having a strong power on the front side of a hybrid type lens in Japanese Patent Laid Open No. 2012-113149, the glass lens for realizing a wider field angle has a large thickness and a small radius of curvature and is difficult to manufacture, and the shape of the hybrid type lens has a large amount of sag and is also difficult to be manufactured.
In Japanese Patent Laid Open No. 1997-015492, Japanese Patent Laid Open No. 2009-063877, Japanese Patent Laid Open No. 2010-107820 and Japanese Patent Laid Open No. 2012-053384, shape restriction is relaxed by the use of an ordinary resin lens or glass lens without the use of a hybrid type lens.
However, in Japanese Patent Laid Open No. 1997-015492, a surface having a small radius of curvature and a strong curvature is used on the image plane side of each lens, and thus tolerance becomes severe, aberration is also easily caused, and compactness is lost. In addition, the correction of the aberration such as color aberration is not sufficient because of a configuration using a single glass material.
In addition, Japanese Patent Laid Open No. 2009-063877 makes great use of a surface having a small radius of curvature for a concave lens or a cemented lens surface, and thus tolerance becomes severe and also aberration is easily caused. Thickness differences among the respective constituent lenses are large and also there exists a lens with a lens thickness almost close to a lens outer diameter thereof, and thus it is not possible to deal with the wafer level lens manufacturing.
Furthermore, in Japanese Patent Laid Open No. 2010-107820, a glass material having a high optical index is used for the first lens and the like, and thus manufacturing cost is increased. In addition, a relatively high power is provided to each lens, and thus aberration changes significantly among the lenses, and tolerance is severe and also aberration is easily caused.
Moreover, in Japanese Patent Laid Open No. 2012-053384, while compactness and low-cost production are considered to be possible, the field angle of a target optical system is small, and when the field angle is tried to be increased, the power of each lens becomes stronger with respect to the power of the whole lenses and also the radius of curvature of a constituent surface becomes small, resulting in a large aberration.